This invention relates to waveguide structures, and particularly though not exclusively to waveguide structures suitable for use as optical sensors.
Sensors which are capable of monitoring biological interactions in real time and with high sensitivity are of considerable importance in life science research. Several sensors exist which monitor changes in the refractive index (or other parameters) of a biological sample, caused by molecular interactions. In a typical sensor an evanescent wave associated with an optical mode existing in a high refractive index dielectric layer of a waveguide extends into a biological sample, which is held in a gel. A change of the refractive index of the sample will modify an optical property of the waveguide mode, and detection of this change will provide dynamic information relating to interactions occurring within the biological sample.
Known optical evanescent sensors include those based on surface plasmon resonance and those based on dielectric waveguiding techniques (see for example Welford, K (1991) Surface plasmon-polaritons and their usesxe2x80x94Optical and Quantum Electronics, 23, 1-27; Smith, A. M. (1987) Optical waveguide immunosensors, Proc. SPIE 798 Fibre Optic Sensors II, 206-213); R. H. Ritchie, Phys. Rev. 106, 874 (1957).
Sensors which use surface plasmon resonance comprise a thin metal layer (typically a few tens or hundreds of Angstroms thick) deposited onto a dielectric prism or grating, and a sensing layer (or a fluid) whose optical properties are of interest provided at an opposite surface of the metal layer. Measurements are made by directing light via the prism or grating onto that side of the metal layer which is not in contact with the sensing layer, and detecting light which is reflected from the same side of the metal layer. A surface plasmon resonance excited by the incident light will result in the absorption of that incident light, and a consequent dip in the reflected light intensity. The condition for exciting a resonance (i.e. the angle of incident light which will excite a resonance) is sensitive to changes in the optical properties of the sensing layer. The optical properties of the sensing layer may be monitored by detecting changes in the angle of incidence which excites a resonance
The resolution, and hence the sensitivity, of sensors which utilise surface plasmon resonance is limited by the resonance width (i.e. the range of angles of incident light which will excite resonance). This width is determined ultimately by the amount of absorption of incident light into the metal layer. Absorption is considerable at wavelengths commonly used for biological measurements, and the maximum resolution of surface plasmon sensors is correspondingly restricted.
The angle of incident light which excites a surface plasmon resonance will alter if the wavelength of the incident light is changed. Variations in the wavelength of incident light will thus introduce an error into measurements. This is a further limitation of surface plasmon resonance sensors, since wavelength-stabilised sources of incident light are needed to allow accurate measurement.
A waveguide structure, based upon the surface plasmon resonance structure and known as a leaky mode waveguide, is described by R. P. Podgorsek, H. Frarke and J. Woods (1998) Monitoring of the Diffusion of Vapour Molecules in Polymer Films using SP-Leaky-Mode Spectroscopy, Sensors and Actuators B-Chemical, Vol.51, No.1-3, pp.146-151. The waveguide comprises a substrate, a thin metal layer disposed on top of the substrate, and a sensing layer whose optical properties are of interest disposed as a further layer on top of the layer of metal. The sensing layer has optical properties which change if the medium is exposed to conditions to be sensed, and may be for example dextran gel.
The leaky mode excited within the sensing layer is of a type known in the art as a bulk mode. This contrasts with the mode which is excited by surface plasmon resonance sensors, which mode is known in the art as a surface mode. Generally only one mode may be excited in surface plasmon sensors (the mode must be a TM mode), whereas the leaky mode waveguide allows the excitation of a series of modes (the modes may be any combination of TE and TM).
A leaky mode of the waveguide, i.e. a bulk mode which is centred on the sensing layer, is excited by directing light towards the layer of metal or metal alloy through the substrate over a range of incident angles. The presence of an excited leaky mode is determined by detecting the intensity of light returned from the waveguide over a range of angles. When light is coupled to a leaky mode of the waveguide this is seen as a dip in the intensity of light emitted from the waveguide. A change of an optical property of the sensing layer will modify the angle of incident light required to excite the leaky mode. The angle at which the dip of intensity is returned from the waveguide will change accordingly.
The leaky mode waveguide is advantageous compared to surface plasmon resonance because a bulk mode of the waveguide is excited rather than a surface mode. This bulk mode is centred on the sensing layer of the waveguide and is therefore considerably more sensitive to changes of the optical properties of the sensing layer than the surface mode provided by surface plasmon resonance.
A disadvantage of known leaky mode waveguides is that detection optics are required to detect a dip in the intensity of light returned from the waveguide, and to follow angular movement of that dip. The absence of light is inherently more difficult to detect than a peak of light intensity.
It is an object of the invention to provide a leaky mode waveguide which will return a peak of intensity when a leaky waveguide mode is excited.
According to a first aspect of the invention there is provided an optical sensor comprising a waveguide having a substrate, a layer of metal or metal alloy disposed on top of the substrate, and a medium disposed as a sensing layer on top of the layer of metal or metal alloy, the medium having optical properties which change if the medium is exposed to conditions to be sensed, the sensor further comprising means for directing light towards the layer of metal or metal alloy through the substrate over a range of incident angles, and detection means for detecting the intensity of light returned from the waveguide over a range of detection angles, the means for directing light being configured to direct light such that a leaky waveguide mode is excited within the sensing layer, and the means for detecting the intensity of light being arranged to detect variations with detection angle in the intensity of returned light resulting from the excitation of the leaky waveguide mode; characterised in that the waveguide is configured such that the overlap of the optical field with the layer of metal or metal alloy is less for light incident at an angle which results in excitation of a leaky waveguide mode than for light incident at an angle which does not result in excitation of a leaky waveguide mode, whereby the detected intensity peakswat a detection angle related to an incident angle which results in excitation of a leaky waveguide mode.
The invention is advantageous because it allows for the easy detection of a waveguide mode.
The term metal alloy is intended to include mixtures of metals and mixtures of two or more elements which include at least one metal. Metals or metal alloys are used because they have a sufficiently high imaginary part of refractive index that an optical field extending into the metal or metal alloy suffers significant loss. The term metal or metal alloy is therefore intended to include any material having an imaginary part of refractive index comparable to that of a metal or metal alloy.
Preferably, the substrate comprises a prism or grating for coupling light into the waveguide mode.
An optical source comprising a laser, a light emitting diode or a source capable of producing a broad spectrum of wavelengths of light may be used to provide the incident light. The use of a light emitting diode, or a broad band source, is made possible by the relative wavelength insensitivity of the waveguide mode of the invention.
The detection means is preferably a charge-coupled-device array (CCD) comprising cells of sufficiently small dimensions to allow resolution of the intensity variations resulting from the excitation of the waveguide mode.
The detection means may comprise a single photo-diode which is capable of being translated across the light returned by the waveguide. By translating the photo-diode through a series of positions, the photo-diode may be made to provide a measurement of intensity at each position, thereby giving a measurement similar to that which will be provided by the CCD array.
The thickness of the layer of medium is preferably greater than 200 nm, and most preferably greater than 300 nm. The layer of medium is required to be thicker than that typically used for surface plasmon resonance sensors, in order to support the waveguide mode which is excited within the medium.
According to a second aspect of the invention there is provided a method of optical sensing comprising providing a waveguide comprising a substrate, a layer of metal or metal alloy disposed on top of the substrate, a medium disposed as a sensing layer on top of the layer of metal or metal alloy, the medium having optical properties which change if the medium is exposed to conditions to be sensed, directing light towards the layer of metal or metal alloy through the substrate over a range of incident angles, and detecting the intensity of light returned from the waveguide over a range of angles, wherein the incident light is directed such that a waveguide mode is excited within the sensing layer, and variations in the intensity of returned light resulting from the excitation of the waveguide mode are detected; characterised in that the waveguide is configured such that the overlap of the optical field with the layer of metal or metal alloy is less for light incident at an angle which results in excitation of a leaky waveguide mode than for light incident at an angle which does not result in excitation of a leaky waveguide mode, whereby the detected intensity peaks at a detection angle related to an incident angle which results in excitation of a leaky waveguide mode.
A disadvantage of conventional waveguides used for optical sensing is that they do not provide an optical mode centred on a sensing layer. This problem is overcome by the leaky mode waveguide.
A limitation of the leaky mode waveguide is that leaky modes are sensitive to changes of the dimensions of the layers comprising the waveguide. The waveguide must therefore be made with tight fabrication tolerances.
It is an object of the present invention to provide a waveguide structure which overcomes or mitigates the above disadvantage.
According to a third aspect of the invention there is provided a waveguide structure comprising a medium disposed as a sensing layer, a second layer of material having a refractive index greater than that of the medium, and a substrate, wherein the structure defines a waveguide capable of supporting an optical mode confined in the sensing layer, the medium is adapted for performing chemical or biological reactions within the medium which will result in a change of an optical property of the sensing layer of the waveguide, and the thickness and refractive indices of the layers are chosen such that an optical mode confined in the sensing layer will suffer substantially anti-resonant reflection as a consequence of the interface between the sensing layer and the second layer and the interface between the second layer and the substrate.
The sensing layer is bounded on one side by a material whose refractive index is lower than that of the sensing layer.
The reference in the statement of invention to a mode being confined in the sensing layer of the waveguide structure is intended to mean that the mode is centred on that layer of the waveguide, and it will be appreciated that a proportion of the mode will extend beyond that layer.
The inventors have realised that anti-resonant reflecting optical waveguides (ARROW""s) may be used to concentrate an optical field in a sensing region having a low refractive index. Since biochemical sample separation, antibody-antigen interactions, etc. are usually carried out in low index layer (dextran gel, a polymer or other suitable medium), ARROW waveguides allow concentration of an optical field in a region in which a chemical or biological reaction is to take place (i.e. the sensing layer of the above waveguide structure).
According to a fourth aspect of the invention there is provided a waveguide structure comprising a medium disposed as a sensing layer, a second layer of material having a refractive index greater than that of the medium, and a substrate, wherein the structure defines a waveguide capable of supporting an optical mode confined in the sensing layer, the medium is adapted for performing chemical or biological reactions within the medium which will result in a change of an optical property of the sensing layer of the waveguide, and the thickness and refractive indices of the layers are chosen such that an optical mode confined in the sensing layer will suffer substantially resonant reflection as a consequence of the interface between the sensing layer and the second layer and the interface between the second layer and the substrate.
The use of a resonant reflection to confine the optical mode, rather than an anti-resonant reflection, is advantageous because it renders the optical mode more sensitive to a change of an optical property of the sensing layer of the waveguide. Waveguides configured to provide an optical mode confined by resonant reflection are hereafter referred to as resonant optical waveguides (ROW""s).
The medium adapted for performing chemical or biological reactions in the ARROW or ROW waveguides is preferably dextran gel, but may be any other suitable low-index material.
Preferably, the ARROW or ROW waveguide structure is adapted for use as part of an optical sensing apparatus.
Preferably, the optical sensing apparatus comprises the waveguide structure, an optical source, means for coupling light from the optical source into an optical mode confined in the sensing layer of the structure, and means for detecting changes in the properties of the optical mode by monitoring properties of light coupled from the waveguide structure.
Preferably, the coupling means comprises a prism which is located against or adjacent the substrate of the waveguide structure, the prism being configured to allow light to be coupled into a resonant optical mode confined in the sensing layer of the structure, when the light is incident upon the prism at a predetermined angle. A change of the refractive index of the sensing layer of the structure will modify the angle which will excite a resonant mode of the waveguide structure
Preferably, the optical sensing apparatus is provided with means for scanning the light from the optical source so that it is incident at the waveguide over a range of incident angles. This may be done for example by mounting the optical source on a swinging arm. In the alternative, means may be provided to direct light from the optical source onto the waveguide from many angles simultaneously.
Preferably, the optical sensing apparatus is provided with means for providing light capable of exciting both TE and TM modes confined in the sensing layer of the waveguide structure, and means for producing interference between light coupled from the TE and TM modes, once it has been coupled out of the waveguide structure.
The optical source used to excite an ARROW waveguide mode may be a light emitting diode, or may be capable of producing a broad spectrum of wavelengths of light. The use of a light emitting diode, or a white light source, is made possible by the relative insensitivity of the ARROW mode index to variations of the wavelength of incident light. If a narrow wavelength band of incident light is required, a laser may be used as the light source. A narrow wavelength band will be required to excite a ROW waveguide mode.
The optical apparatus may include means for detecting a dip in the intensity of the light coupled from the waveguide. Should the waveguide structure cause scattering or absorption of light confined within the sensing layers a dip in the intensity of light coupled from the waveguide will indicate the presence of a waveguide mode.
The waveguide structure may be provided with a low index spacer layer located between the second layer and the substrate. The low index spacer layer is advantageous because it allows ARROW modes and resonant mirror modes to be excited in a single waveguide, thereby allowing comparison between them. The low index spacer may similarly allow the simultaneous excitation of ROW modes and resonant mirror modes.
The waveguide structure may be arranged to cause scattering or absorption by the introduction of scattering or absorbing elements in the sensing layer or the second layer of the waveguide, or by providing either of those layers with roughened surfaces. Where the waveguide structure includes a low index spacer layer. scattering or absorbing elements may be introduced into the spacer layer. The spacer layer may be provided with roughened surfaces.
The waveguide structure may be provided with a further layer spaced apart from the second layer by a layer of lower refractive index, the further layer having a refractive index greater than that of the sensing layer. The introduction of this extra layer will decrease the losses suffered by a mode confined in the first layer of the waveguide, and will decrease the range of angles of incident light which may be used to excite a resonant mode confined in the sensing layer of the waveguide structure.
The waveguide structure may be provided with a fourth layer located on an uppermost surface of the sensing layer, the fourth layer being material with a similar refractive index to the second layer, and a fifth layer of substrate located on top of the fourth layer. The sensing layer will thus effectively be bounded on both sides by ARROW or ROW structures. This structure may be referred to as a symmetric ARROW structure or symmetric ROW structure, although the corresponding layers on either side of the sensing layer are not required to be of identical thickness or to have the same refractive index. In this configuration, the sensing layer may consist of a fluid that may be allowed to flow through the waveguide structure. This configuration allows an optical mode to be confined in the fluid, and thereby allows the properties of the fluid to be monitored.
Optical sensing apparatus for use with a waveguide comprising the above symmetric waveguide structure may include means for detecting a dip in the intensity of the light coupled from the waveguide. Resonant modes of the waveguide will be manifest as dips in the intensity of light reflected from the waveguide structure or peaks in the intensity of light transmitted by the waveguide structure
The optical apparatus may be configured to detect the presence of gases or chemicals suspended in the air, water or other fluid. One way in which this may be done is by forming the sensing layer of the waveguide structure from a polymer or other material whose refractive index, density or other property is sensitive (i.e. altered) by the presence of that chemical or biochemical species that is to be detected.
The optical apparatus may be arranged to monitor changes of the refractive index of the sensing layer of the waveguide structure, or alternatively the apparatus may be arranged to monitor fluorescence or absorption within the sensing layer.
According to a fifth aspect of the invention there is provided a method of optical sensing, comprising coupling light into a mode confined in the sensing layer of a waveguide structure described in accordance with the third aspect of the invention or the fourth aspect of the invention, coupling light out of the waveguide structure using a prism, and monitoring the angle at which coupling of light to the mode passes through a resonance.
The method may include coupling white light into a mode confined in the sensing layer of the waveguide structure described in accordance with the third aspect of the invention, thereby allowing the spectroscopic analysis of biological samples.
It is an object of the present invention to provide an alternative waveguide structure which supports an optical mode centre on a sensing layer.
According to a sixth aspect of the invention there is provided a waveguide comprising a sensing layer of a medium, a second layer forming a lower surface of the medium and having a refractive index greater than that of the medium, and a third layer forming an upper surface of the medium and having a refractive index greater than that of the medium, wherein the medium is adapted for performing chemical or biological reactions within the medium which will result in a change of an optical property of the sensing layer of the waveguide, and the waveguide is capable of supporting an optical mode centred on the sensing layer.
The waveguide, which will be referred to as a light condenser, is advantageous because its structure is very simple, and it is robust with respect to environmental changes (for example temperature fluctuations).
The light condenser mode is centred on the sensing layer, thereby providing sensitive measurement of changes of the optical properties of the medium comprising the sensing layer.
According to a seventh aspect of the invention there is provided a waveguide comprising a sensing layer of a medium, a second layer forming a lower surface of the medium and having a refractive index greater than that of the medium, and a third layer forming an upper surface of the medium and having a refractive index less than that of the medium, wherein the medium is adapted for performing chemical or biological reactions within the medium which will result in a change of an optical property of the sensing layer of the waveguide, and the waveguide is capable of supporting an optical mode centred on the sensing layer
The waveguide according to the seventh aspect of the invention provides a light condenser reflection at the interface between the layer of medium and the second layer, and conventional total internal reflection at the interface between the layer of medium and the third layer.
A known construction of optical sensor, referred to as a resonant mirror biosensor, attempts to combine the sensitivity of waveguiding devices with the simple construction and use of surface plasmon resonance devices (see Cush, R. et al (1993) The resonant mirror, Biosensors and Bioelectronics, 8, 347-353). The resonant mirror biosensor is similar in construction to a surface plasmon resonance device. A sensing layer, i.e. the material whose optical properties are to be monitored, is placed in contact with a high refractive index layer. The refractive index and thickness (typically about 100 nm) of the high index layer are selected in such a way that the sensitivity of the sensor is maximised. This high index layer is separated from a prism by a layer of lower refractive index material, called the spacer layer (e.g. silica). The refractive index and thickness (typically about 0.5 microns) of the lower index layer are selected such that the sensitivity of the sensor is maximised and/or the sharpness of the Resonant Mirror resonances are maximised. The sensitivity of the sensor and sharpness of the modes can be controlled by altering the refractive index or thickness of the high index layer and spacer layer. The refractive index of the prism also controls the sensitivity of the sensor and sharpness of the modes. The refractive index of the prism must be higher than the mode index of the Resonant Mirror modes.
The resonant mirror differs from conventional waveguide sensors in that the mode excited in the waveguide sensor is leaky in nature. This feature, which may also be seen in surface plasmon resonance waveguides, allows light to be coupled into and out of the resonant mirror via the prism.
Efficient coupling of light to the high index dielectric layer occurs only for certain angles of incident light where phase matching between an incident beam and resonant modes of the high index dielectric layer is achieved. At a resonant point, light couples into the high index dielectric layer and propagates some distance along the sensing interface before coupling back into the prism. An evanescent wave associated with the resonant modes of the high index dielectric layer will extend into the sensing layer. Changes of optical properties of the sensing layer will alter the properties of the resonant modes of the high index dielectric layer. Generally, the thickness of the high index dielectric layer is made very low, in order to maximise the proportion of the optical mode in the evanescent field interacting with the sensing layer, and so maximise the sensitivity of the device. The thin waveguiding layer generally provides a single waveguide mode (one TE mode and/or one TM mode).
Leaky resonant mirror modes in the resonant mirror biosensor may exist for both TE and TM polarisations, and are seen as fine structure in the reflected light once it has passed through an output analyser. The angles of incidence which excite modes of the high-index layer are sensitive to changes in the sensing layer, and so changes caused by assay reactions in the sensing layer may be monitored by measuring shifts in the excitation angle.
A limitation of resonant mirror waveguides is that a variation in the wavelength of light incident at a waveguide will alter the angle of incidence required to excite resonant modes of that waveguide. The effect of a change of incident wavelength cannot be separated from the effect of a refractive index change in the sensing region, and the sensitivity of an optical sensor comprising the resonant mirror is thus limited by the extent to which variations of the wavelength of incident light can be suppressed. Lasers are used to provide the narrow wavelength band of night required for resonant mirror optical sensors. Unfortunately, lasers are susceptible to an effect known as xe2x80x98mode hoppingxe2x80x99 wherein the laser wavelength jumps between different values which satisfy the resonance, criteria of the laser structure. The wavelength produced by a laser will also vary with temperature due to variation of the dimensions of that laser. Known resonant mirror optical sensors attempts to minimise the wavelength variations in the output of a laser by providing a wavelength stabilisation mechanism. However, this mechanism is both complex and expensive.
Optical sensors comprising other optical waveguides structures may also be susceptible to wavelength changes.
It is an object of the present invention to provide a waveguide structure which overcomes or mitigates the above disadvantage.
According to an eighth aspect of the invention there is provided an optical sensor comprising a waveguide defined by a plurality of layers including a sensing layer comprising a sensing medium adapted for performing chemical or biological reactions which will result in a change of an optical property of the sensing layer, the layers being capable of supporting at least one optical mode, wherein at least a first component of a supported mode extends into the sensing layer to a substantial extent such that the first component is affected by changes in optical properties of the sensing layer, and at least a second component of a supported mode does not extend into the sensing layer to a substantial extent such that the second component is not substantially affected by changes in optical properties of the sensing layer, the sensor further comprising means for detecting variations in signals representative of the first and second components, and means for comparing the detected signals to identify variations which substantially affect only the first component.
The optical sensor is advantageous because the second component will be substantially unaffected by the optical properties of the layer of sensing medium, and may be used as a reference component. The first component will be affected substantially by the optical properties of the sensing layer, and may be used to measure said optical properties. An unwanted experimental variation, for example a change of wavelength of light coupled to the waveguide, since it will affect both components equally, may be removed from a measurement of the optical properties of the medium by comparison of the measurement and reference components.
Suitably, the layers are capable of supporting two modes a first of which is the first component and a second of which is the second component.
Preferably, the two modes are centred on different layers of the waveguide.
The two modes may be resonant mirror modes. Alternatively, the two modes may be anti-resonant reflecting optical waveguide (ARROW) modes, resonant optical waveguide (ROW) modes or light condenser modes. Other different forms of modes may be supported.
The optical sensor is advantageous for measurements utilising modes other than resonant mirror modes for the same reasons given above in relation to resonant mirror modes.
An optical sensor, according to the invention, which is designed to support resonant mirror modes may have a sensing layer of semi-infinite thickness, or may have a sensing layer of finite thickness. In contrast to this, an optical sensor which is designed to support ARROW modes or ROW modes must have a sensing layer of finite thickness, and cannot have a semi-infinite sensing layer.
The layers may be capable of supporting a single mode, a first portion of the single mode defining the first component which extends into the sensing layer, and a second portion of the single mode defining the second component which does not extend into the sensing layer.
According to a ninth aspect of the invention there is provided a method of optical sensing comprising exciting at least one optical mode in a waveguide structure defined by a plurality of layers including a sensing layer comprising a sensing medium adapted for performing chemical or biological reactions which will result in a change of an optical property of the sensing layer, wherein at least a first component of a supported mode is excited so as to extend into the sensing layer to a substantial extent such that the first component is affected by changes in optical properties of the sensing layer, and at least a second component of a supported mode is excited so as not to extend into the sensing layer to a substantial extent such that the second component is not substantially affected by changes in optical properties of the sensing layer, the method further comprising detecting variations in signals representative of the first and second components, and comparing the detected signals to identify variations which substantially affect only the first component.
Preferably, two modes are be supported by the layers, a first of which is the first component and a second of which is the second component. The two modes may be centred on different layers of the waveguide.
The two modes may be resonant mirror modes, or anti-resonant reflecting optical waveguide modes
A single mode may be supported, a first portion of the single mode defining the first component which extends into the sensing layer, and a second portion of the single mode defining the second component which does not extend into the sensing layer.